The present invention relates to a method for fabricating a semiconductor device and, more particularly, to a method for fabricating a trench metal-oxide-semiconductor field effect transistor (MOSFET).
In general, a trench MOSFET includes a semiconductor substrate having a trench formed between a source and a drain of the trench MOSFET. The trench MOSFET includes a perpendicular channel due to the presence of the trench, thereby extending a gate between the source and the drain.
The trench described above is filled with a conducting material, such as polysilicon, so that a low current flow is possible in the trench MOSFET, thereby providing a low value of specific on-resistance.
After a thin insulating film, such as an oxide film, is formed on inner walls of the trench to form a gate insulating film, the polysilicon is deposited over the trench and a hard mask is formed on the semiconductor substrate. Then, the polysilicon fills in the trench and covers the oxide film.
Thereafter, the polysilicon is etched by means of an etch back process, which directly etches the polysilicon without using a photo resist film as an etch mask, so that a gate electrode formed of the polysilicon is formed in the trench. Here, in the etch back process of the polysilicon, an over-etching operation is performed. The over-etching operation further etches the polysilicon for some time even after the hard mask, which is used as an end point detection film, is exposed. At this time, the polysilicon remains only within the trench, and the gate electrode is formed. The gate electrode thus formed has an upper surface lower than that of the semiconductor substrate.
If the gate electrode does not fill the trench due to over-etching of the polysilicon, the capacitance of a gate may become smaller than a desired value. Consequently, there may be an insufficient amount of current flowing through the channel of the trench MOSFET.
In addition, since the upper surface of the polysilicon may be considerably damaged when the etch back process proceeds, the gate electrode formed in the trench must be thermal-processed for some time in a subsequent process.